138 WALTER GARSTANG, M.A. 



ninth horizontal bar, but at the sixth bar its left lip suddenly thins 

 out and bends over the right lip, concealing it from view, and con- 

 tinuing posteriorly as the dorsal lamina. This structure has a very 

 characteristic form in this species (PL VI. fig. 10). It is moderately 

 deep, provided with a regular succession of transverse ribs on its 

 convex side and of well-marked teeth on its edge, the latter correspond- 

 ing to the number of ribs. There are no intermediate pectinations 

 of its edge ; but the concave side instead of being smooth as is usually 

 the case in Ascidians, is profusely studded with stout papillae, as 

 shown in the figure. There is a certain tendency of the papillte to be 

 arranged in rows directed obliquely from the summit to the free edge 

 of the lamina, from before backwards ; but this general tendency is 

 frequently departed from. The dorsal lamina is continued for some 

 distance behind the oesophageal aperture. 



Branchial apparatus. — The horizontal vessels are often of two 

 sizes,* broad and narrow, and these vessels alternate with one another 

 in position ; but the breadth of the larger vessels never exceeds half 

 the antero-posterior diameter of the meshes — usually it is considerably 

 less. The pharyngeal wall is minutely plicated. The internal longi- 

 tudinal bars are slender in form. At their junctions with the 

 connecting ducts are situated blunt papilla? of characteristic shape ; 

 they are of an erect discoid form, with a semi-circular edge, com- 

 pressed from before backwards, and provided with a supporting ridge 

 or buttress upon their anterior and posterior faces (fig. 11 b). Usually 

 the meshes are square, and intermediate papilla? quite absent ; but in 

 some parts of the pharynx transverse rows of meshes may frequently 

 be observed which are distinctly elongated in a longitudinal direction, 

 and in such regions minute intermediate papilla? may be detected upon 

 the internal longitudinal bars. The elongation of the meshes and 

 appearance of intermediate papilla? is preparatory to the formation of 

 a new series of horizontal vessels, in the manner which I have 

 described above in Ascidia mollis. There are four or five stigmata in 

 each mesh (fig. 1 1 a). There is no pharyngo-cloacal slit. 



My largest specimen is mature, and minute white ova are present 

 in the oviduct. 



* This distinction of size is much less apparent in mature than in young 

 individuals. 



